The present invention relates generally to a method and apparatus for preparing high-purity metallic silicon, such as is used in solar cells. More particularly, the invention relates to a method and apparatus for efficiently and economically preparing high-purity metallic silicon from powdered silicon dioxide.
Conventionally, metallic silicon or ferrosilicon has been prepared in arc furnaces from a mixture of silicon dioxide and carbon. In order to ensure proper ventilation and to improve the efficiency of the reduction reaction in the high temperature region of the furnace, it has been considered essential to use large grains or masses of silicon dioxide.
On the other hand, there has recently been an increasing demand for metallic silicon of high purity, especially that having a purity higher than 99.999%. Such high-purity metallic silicon has numerous applications, among which are solar cells. Refined natural silica is commonly used as the source of silicon dioxide. This refined silica is generally powdered or granular with a grain size of less than several milimeters. Therefore, in order to use this refined silica as the source silicon dioxide in the conventional apparatus, an extra process step is needed to sufficiently increase the grain size of the refined silica. This obviously leads to higher production costs and may also lower the purity of the source material.
In order to eliminate these problems in the prior art, an improved process for preparing high-purity metallic silicon has been proposed in Japanese Patent First Publication (Tokkai) Showa 57-11223. However, even the proposed improved process still requires that a part of the source silicon dioxide charge have a grain size of 3 to 12 milimeters.
Another improved process of preparing high-purity metallic silicon has been disclosed in the Japanese Patent First Publication (Tokkai) Showa 58-69713. In the disclosed process, the reaction between silica and carbon takes place in a high-temperature plasma jet which transports the resultant product onto a carbon layer. In this proposed process, a large amount of silicon carbide is created as a result of reaction with the carbon layer. The created silicon carbide tends to accumulate within the carbon layer and fill interstices between the carbon grains, which inhibits further reaction. Due to this defect, the process proposed in the Japanese Patent First Publication 58-69713 is still not useful in the manufacture of high-purity metallic silicon.